Threat Description

Worm:​W32/Welchi

Details

Aliases:Sachi, WORM_MSBLAST.D, Nachi, Welchia
Category:Malware
Type:Worm
Platform:W32

Summary



A standalone malicious program which uses computer or network resources to make complete copies of itself. May include code or other malware to damage both the system and the network.



Removal


Automatic action

Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.

More

You may wish to refer to the Support Community for further assistance. You also may also refer to General Removal Instructions for a general guide on alternative disinfection actions.

For more general information on disinfection, please see Removal Instructions .

Network Disinfection

For general instructions on disinfecting a local network infection, please see Eliminating A Local Network Outbreak.

Manual Disinfection

Caution: Manual disinfection is a risky process; it is recommended only for advanced users.

The worm will stop working when the computer's clock is set to 2004. This feature can be used to remove the worm from the computer. This can be automated with the date and the shutdown commands, for example:

  • DATE 01-01-2004
  • SHUTDOWN -r


Technical Details



Worm:W32/Welchi is an unusual malware in that it attempts to disinfect the computer system from Worm:W32/Lovsan infections. It also attempts to patch a vulnerability used by the Lovsan worm to propagate.

The virus code contains the following text string, which is never displayed:

  • =========== I love my wife & baby :)~~~ Welcome Chian~~~ Notice: 2004 will remove myself:)~~ sorry zhongli~~~==== ======= wins

Welchi was first reported on August 18th 2003. Tthe worm will stop working when the computer's clock is set to 2004.

Infection

Welchi only infects Windows XP machines through the RPC hole used by the Lovsan worm.

In addition, Welchi will attempt to infect IIS 5.0 web servers via WebDAV exploit. For more on this vulnerability found in March 2003, see http://www.microsoft.com/technet/security/bulletin/MS03-007.asp.

This variant uses files named DLLHOST.EXE and SVCHOST.EXE. Note that these are names of normal Windows system files.

On infection, the worm drops files at the following locations:

  • %systemDir%\wins\DLLHOST.EXE - The worm's executable file
  • %systemDir%\wins\SVCHOST.EXE - The renamed tftp server

Activity

Unlike most worms, Welchi's effects on the infected system appear to be beneficial. When active, it attempts to disinfect Lovsan.A from the machine by looking for a process with the strings "msblast" in its name (a Lovsan characteristic) and terminating the process if found.

It will then delete the file %systemdir%\msblast.exe, Lovsan's executable copy.

Welchi also attempts to download the Microsoft patch for closing the RPC vulnerability, and tries to download the patch from eight different URLs, corresponding to four language versions (English, Chinese, Simplified Chinese and Korean) for both Windows XP and 2000. This patch routine is not always successful and on clearing Welchi from the machine it is recommended that the patch be checked according to instructions from Microsoft (see the section File Information at http://support.microsoft.com/?kbid=823980).

Registry

When run, Welchi will create a mutex named "RpcPatch_Mutex", to ensure that only one active copy of the worm is ever run.

The worm does not modify the Windows Registry to launch itself automatically.






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